Twelve-year-old Keith Bennett is the only victim of Ian Brady and Myra Hindley whose body has never been found.

His family have never had the closure that the other victims' parents had, if finally filling the void left by the killing in 1964 can be deemed thus.

For the past 15 years Keith Bennett's brother, Alan, who was eight when his brother vanished, has searched and re-searched vast tracts of Saddleworth moor seeking an end to the unresolved grief. It never came.

The killing of Keith Bennett remained stubbornly unacknowledged by Brady and Hindley for more than 20 years, Brady seemingly enjoying the vestige of control the knowledge gave him. It was only in 1985 that Brady finally confirmed that he and Hindley had murdered the boy, and Pauline Reade, as well as the three other children they were convicted of killing.

Last month, Mr Bennett told the Guardian: "At first we used to imagine that he'd been kidnapped, then gradually you realise that he's not coming back. Ian Brady took him to a place of his choosing, where he murdered and buried him. I just can't bear it. I know it will never bring him back to life, but finding Keith and burying him is a way of putting it all right, bringing it to an end."

The family will be hoping that Brady's sudden offer to find Keith's grave is genuine.

In a letter from his solicitor to Mr Bennett, Brady writes: "Speed is essential in the circumstances, not talk. This is my final effort."

It follows moves by Hindley earlier this year to help find the body. Through a documentary maker, Duncan Staff, she supplied a series of maps to forensic archaeologists working on the case.

One sketched two streams, thought to be Shiny Brook and Hoe Grain, and a series of small arrows meandering to where the boy was led to his death. The search has yet to find the grave.

The team, which is led by John Hunter of Birmingham University, has carried out a detailed geophysical survey of the area Hindley highlighted. They have also studied aerial photographs taken over the past 50 years. These show that the areas of exposed peat, where it would have been possible to bury a body, have shifted by up to 20m since 1964.

"This is information the police did not have when they searched the moor in the mid 1980s and means they could have been looking in the wrong place," said Mr Hunter.

The Birmingham University team have carried out a number of exploratory excavations, but have had to stop work for the winter. Brady's offer to help, which he describes as "altruistic" was made in response to a request by Mr Bennett.

Brady, who is in the third year of a hunger strike and is fed through a plastic tube, makes a number of conditions: the case must be handled by Yorkshire police, rather than the Greater Manchester force, which he accuses of incompetence.

He also says that he will only help if no members of staff from the psychiatric hospital where he is being held are present. He has twice brought high court cases against the hospital, demanding the right to die. "My return to Saddleworth must be in police custody, as before, but with no Ashworth staff at all."

"I have never applied for parole and never will," he says. Brady also says that he will only go back if Mr Hunter is present as "an independent witness".

Mr Hunter said that, given Brady's track record, his offer needed to be treated with caution.

"He has been back twice before with the police and did not provide them with any useful information. However, it is possible that the change in landscape could have confused him and, in the end, there are only two people who know where Keith is buried so you cannot dismiss this."